The W-MODE plugin for launchers like ChaosLauncher will make StarCraft run in a window. Works perfectly, even under 64-bit Windows 7.
With this plugin, you can lock the mouse cursor inside the game window to avoid accidentally moving the mouse outside when trying to scroll, you can lock the window to prevent it from being moved, and you can have it double the window size to make the game take up more of the screen (which may be helpful, since Starcraft runs at a pretty low resolution and it may be hard to see things otherwise).